1. Field of the Invention
The present invention relates to power control method for forward links and to a structure of a mobile station and a base station for the control in Code Division Multiple Access (CDMA) system, and more particularly, to a power control method for forward links in the CDMA system which can increase forward link capacity by requesting power increase/decrease assigned to corresponding base stations as a mobile station measures signals of forward traffic channels from each base station in soft handoff and a base station adjusts transmit power of corresponding traffic channels depending on the state of forward links.
2. Description of the Prior Art
Typically, the quality of forward traffic channels measured by a mobile station is reported to a corresponding base station periodically or only when the value excesses a threshold value, for power control of forward links in the CDMA cellular mobile communication system, and then the base station to receive the report adjusts transmit power assigned to the corresponding traffic channel. Also, when a mobile station exchanges call information with multiple base stations, the station sends the same measured information to the above multiple base stations, not dividing the measured quality information. Namely, a mobile station may receive all signals of base stations because all base stations send signals by the same frequency in the CDMA cellular mobile communication system. Accordingly, when 2 base stations send the same traffic information to a certain mobile station, it is possible that the mobile station demodulates the received signals, using the diversity combining. When the same information is sent from corresponding 2 base stations to the mobile station under handoff until its completion after starting handoff from one base station to the other base station, the mobile station may demodulate them after receiving and combining the signals of both base stations. The mobile station may not only measure the pilot signal strength from home base station, but it can measure pilot signals of neighboring base stations at the same time. As a result, the mobile station starts handoff by exchanging the same traffic information with the base station under conversation and the corresponding base station simultaneously if the receiving pilot signal strength of a base station is larger than a specified value. Then, the mobile station completes soft handoff by demodulating only the information sent by the base station whose receiving strength of pilot signal is the largest and requesting to stop communication to another base stations when the station decides that the receiving strength of pilot signal of a certain base station is enough larger than the strength of pilot signals from another base station and its handoff is completed.
A mobile station may improve receiving performance by performing the multipath diversity combining of multiple paths, using separate multipath combining receivers such as a rake receiver in addition to 2 receivers under normal operation. At this time, the station receives signals from another base stations in handoff using a part of receivers having been used as multipath combining receivers under normal operation, not using the redundant receivers which may be used from the soft handoff start until its completion.
On the other hand a mobile station reports the information to the base station under conversation by receiving signals of forward links and measuring traffic channel quality with the described method for power control of forward links if normal, and then the base station, based on the information, increases/decreases transmit power of forward traffic channels for the corresponding mobile station. However, the mobile station reports the quality of the measured forward traffic channels to the corresponding base stations for power control of forward links during soft handoff, but also reports the measured quality the same to the base stations engaged in current handoff, without considering the state of forward links of the base stations under current handoff. As the result, the base station increases or decreases transmit power of forward traffic channels of all base stations connected to the mobile station under conversation by the same type, based on the reported information.
When power control of forward traffic channels is performed by such a method, the transmit power of forward traffic channels of base stations, in the base stations engaged in current handoff, whose forward link is at a good state also increases. Accordingly, it causes more interference signals to neighboring mobile stations and forward link capacity decreases.
Accordingly, a power control method for forward links in a CDMA cellular mobile communication system in accordance with the present invention comprises a step to compare the combined signal-to-noise ratio of a forward traffic channel measured by a mobile station under normal operation during conversation with a target value for power control for forward links, and a step to decide whether to request transmit power increase or decrease of a corresponding forward traffic channel when the combined signal-to-noise ratio is smaller than the target value.
Also, to achieve the above object, power control of forward links in soft handoff in the CDMA cellular mobile communication system in accordance with the present invention is characterized by comprising a step to compare the combined signal-to-noise ratio of a forward traffic channel received from base station during conversation by a mobile station under soft handoff with a target value for power control, a step to request for decreasing the sending power of the forward traffic channel as much as a fixed amount to all base stations engaged in current handoff when the combined signal-to-noise ratio is larger than the target value, and a step to request for increasing the transmit power of the forward traffic channel as much as a fixed amount only for the base station whose receiving pilot signal-to-noise ratio measured from pilot channel that transmitted from each base station is the largest and to request for decreasing the transmitting power of the forward traffic channel as much as fixed amount for the other base stations, when the combined signal-to-noise ratio is smaller than the target value, so that it may be possible to divide the transmit power request information for each base station.
Also, to achieve the above object, a power control method for forward links in a CDMA cellular mobile communication system further comprising a step to increase or decrease the sending power of the forward traffic channel to each base station in accordance to the each divided transmit power request information for each base station.
Additional features and advantages of the invention will be set forth in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.